Growth and characterization of high quality CuInSe₂ epitaxial thin films on GaAs substrates

Abstract

CuInSe₂ (CIS) and its alloys are the important materials that have been used as an absorber layer in high efficiency thin film solar cells. The understanding in its physical properties and growth mechanism are very essential for material improvement and the reproducibility is necessary in this study. The epitaxial growth is thus chosen for this work. The CIS epitaxial films are grown on GaAs (001) substrates by molecular beam epitaxy technique in various compositions and growth processes. Reflection high energy electron diffraction (RHEED) and pyrometer signals are applied as the in-situ monitoring tools in this growth. The results show that the Cu-rich and near stoichiometric films display the streaky RHEED patterns unlike the Cu-poor films. The Cu-rich films are the combination phases between the CIS compounds and the excess Cu-Se phase that can be removed by KCN aqueous solution and is dependent upon the substrate temperature. The best phase of Cu-Se that plays the important role in the high quality films is the Cu₂₋ₓSe which is the main cause of the undulation structure and rectangular-shape protrusions in the two-stage films. The excess Cu₂₋ₓSe can reside everywhere in the growing films and results in the presence of CuGaSe₂ interface layer that disappears when the substrate temperature is below 430℃. The growth model of the Cu-rich CIS epitaxial films can be proposed. The mechanism of the two-stage growth process that is evolved from the Cu-rich films which demonstrates the high quality precursor is the incorporation of In and the excess Cu-Se compounds. The mechanism of the proposed modified two-stage can enhance the structural quality of the films. The epitaxial films grown under the Cu-rich conditions present the high crystallinity whereas the other conditions reveal the many defect complexes.